The overall goal of our proposed research is to understand the mechanisms that allow zoonotic, sylvatic arthropod-borne viruses (henceforth referred to as 'arboviruses') to jump species boundaries and cause human disease. Dengue viruses (DENV) are by far the most important human pathogens, causing an estimated 50- 100 million annual infections in tropical and subtropical regions, where nearly a third of the global population is at risk. The four serotypes of DENV that circulate among humans each emerged from an ancestor that is maintained in a sylvatic transmission cycle between non-human primates and arboreal mosquitoes in Southeast Asia. This project will examine the vector ecology and mechanisms of DENV emergence from its sylvatic transmission cycle in Malaysian Borneo. Discerning the underlying mechanisms of dengue emergence will provide key insights into the epidemiology and risk of arboviral infections in Malaysia; our findings may also lead to the preparation of guidelines for arbovirus surveillance, control and outbreak management for the entire Southeast Asian region. Critically, the sylvatic transmission cycle contains virus variants that have not yet made the transition into the human cycle; we have recently discovered a fifth DENV serotype from the sylvatic cycle in Borneo. The identification of DENV-5 and perhaps additional sylvatic lineages could have profound implications for vaccine development and diagnostics. We therefore propose to prospectively investigate the nature and breadth of contact between humans and the sylvatic DENV transmission cycle, as well as the consequences of this contact in terms of human dengue disease. Our research will focus on the documented sylvatic transmission cycle in Sarawak, Malaysian Borneo The following hypotheses will be tested: (i) sylvatic DENV circulates continuously in the forests of Sarawak; (ii distinct suites of vector mosquitoes are responsible for maintenance of the sylvatic cycle in reservoir hosts and spillover into humans, (iii) Borneo, which has been separated from mainland Asia for approximately 12,000 years, contains DENV variants not found in either the human cycle or the sylvatic DENV cycle of mainland Malaysia (iv) both traditional practices, such as hunting and slash-and-burn agriculture, that necessitate entry into the forest as well as large-scale anthropogenic changes in land use constitute risk factors for sylvatic DENV infection and disease. The ICIDR consortium will include the University of Texas Medical Branch, the Universiti Malaysia Sarawak, and New Mexico State University. The projected outcomes of this proposal will broaden our understanding of sylvatic vector ecology and the mechanisms by which DENV and other arboviral pathogens emerge, thus providing a direct benefit to human health. Our results will also improve predictions of the risk of future emergence of DENV and other arboviruses, and result in the identification and characterization of various arboviral etiologies of febrile illness in Borneo where little is known about the zoonotic disease burden attributable to arboviruses. Thus, it will improve the clinical recognition of arbovirus illness at rural localities in Malaysia.